The emitted light quantity of an LED depends on its temperature. In laboratory applications according to prior art, according to which an LED is employed as reference light source, the LED and possibly the measuring apparatus associated therewith are tempered, resulting in the temperature and, thus, the emitted light quantity of the LED remaining constant.
In applications outside the laboratory, in which such a climatisation is not possible at all or only at increased expenditure, it is therefore necessary to correct the measured values of the light quantity with respect to the temperature contingent influences, to thereby reduce the errors of the measured result. In case such an LED is used for example as light source for stabilization of a photo multiplier, which for example is employed as light detector in a scintillation detector, for example a mobile detector for identification of radio isotopes (hand held radio isotope identification device—RID), the LED is exposed to thermal fluctuations in the range of −20° C. to +50° C. Thereby, the system amplification of the light detector can fluctuate offhand for about 20% and more, such that a stabilization of the amplification of the light detector is necessary, to maintain the energy amplification and the energy resolution of the RID sufficiently good. For stabilization of such a light detector with an LED, it is therefore necessary, to know the temperature dependency of the light quantity emitted by the LED.
Methods for stabilization are known, according to which the temperature is measured at or in the detector and the temperature caused effects are adjusted by means of previously measured calibration tables. These methods, however, have the drawback that a temperature measurement with fast temperature changes is only hardly realizable, particularly for the reason that often no uniform temperature distribution can be expected in the detector. Besides, the amplification of, for example, a photo multiplier does not only depend on its temperature, but rather also on the effective counting rate and its previous history, i.e. its hysteresis and age. It has been found that sufficiently exact prediction of the amplification under consideration of all parameters is not possible.
For stabilization, therefore, often active methods are employed during the actual measurement. Mostly, radio active calibration sources or natural background radiation are used, to achieve such an active stabilization. This, however, leads to optimization problems, because a compromise of sufficiently short but nevertheless sufficiently exact calibration measurements has to be found. Additionally, each additional radio active radiation leads to a reduction of the total sensitivity of the system.
An alternative is the separated stabilization of light detector and scintillator—the latter is for example disclosed in PCT/EP2004/050754. It is known to use a pulsed light source, for example an LED, as measured standard for the stabilization of the light detector. It is also known to stabilize and to monitor the amplification of light detectors in this manner in laboratory applications. Disadvantageous with respect to this prior art is that the light emission of an LED depends on its temperature, more particular, on its junction temperature TLED. Thus, according to known methods, it is either necessary, to keep the temperature constant or to monitor it at least, or to monitor the light quantity emitted respectively by the LED with a separate measurement apparatus precisely. Such an assembly is not only technically complex and cost intensive, but rather requires also additional energy and additional space, complicating the use in battery operated mobile RIDs.
From sensor techniques, a method is known, to measure the temperature of semiconductor elements by means of a current measurement at constant operating voltage or by means of a measurement of the flux voltage at constant current.
EP 1,283,569 A2 discloses the correction of the light emission of an LED as a function of the temperature and by using a characteristic parameter curve. The disadvantage of this method is, that an additional element, namely a temperature sensor is necessary.
EP 1,039,597 A2 describes the stabilization of the light emission of an LED on the basis of the measured current and voltage at the LED itself. A stabilization of the effective light emission does not take place so that substantial uncertainties remain. In addition, only certain LED's may be used at all.
U.S. Pat. No. 4,160,165 is discussing the problem of stabilization of a x-ray detector, being operated in a pulsed mode, so that the output current of the photomultiplier is kept constant during the pulsed operation. This invention is designed to correct for background noise during the x-ray measurement. Corrections of temperature dependencies or even an energy stabilization are not an object oft this disclosure.